Beaufort vs Vigenère — two keyword ciphers, one subtraction apart

Beaufort and Vigenère are so alike they are often confused: same 26×26 table, same key repeated under the plaintext. The difference is a single operation — and it changes how you use them.

Add or subtract?

Vigenère adds: the ciphertext letter is (plain + key) mod 26. To decrypt, you subtract the key.

Beaufort flips the direction: the ciphertext letter is (key − plain) mod 26. In practice you find the plaintext letter in the key’s row and read off the column header.

Beaufort’s edge: it is its own inverse

Because Beaufort subtracts the plaintext from the key, applying the same operation twice returns the original text. Encryption and decryption are therefore identical — one mode, one table, no inversion to remember. This is exactly the principle behind the cylinder machine that bears the name.

Vigenère, by contrast, needs two distinct procedures (add to encrypt, subtract to decrypt).

Same key, same weakness

Security-wise the two are equivalent. Both rely on a repeated keyword, so both fall to the Kasiski examination and index-of-coincidence analysis: recover the key length from repetitions, then reduce the whole thing to L frequency analyses. Beaufort adds no extra strength — only operational convenience.

When to use which

• Teaching, demonstrating the polyalphabetic principle → Vigenère. It’s the name everyone knows and addition is intuitive.
• Hand encryption where you want a single gesture → Beaufort. No risk of mixing up direction between encrypting and decrypting.
• Real security → neither. AES-256 or ChaCha20.